Update release instructions for mingw makefile.

Have successfully automated the build, but packaging
is still manual.

doc/release.txt

@@ -23,21 +23,10 @@ to https://archive.mozilla.org/pub/opus/
 Win32 binaries:
 
 - Assuming the mingw cross toolchain:
-- Checkout/download latest or other appropriate
-  versions of libogg, opus, and any other supported
-  dependencies.
-- Compile them with:
-    ./configure --host=i686-w64-mingw32 --prefix=/path/to/builddir
-    make && make check && make install
-  or similar. Wine is required for 'make check' to complete.
-- Compile openssl with:
-    ./Configure shared mingw64 no-asm --prefix=/path/to/builddir \
-      --cross-compile-prefix=i686-w64-mingw32-
-    make depend && make -j8 && make install
-- Compile opusfile with:
-    ./configure --host=i686-w64-mingw32 --prefix=/path/to/builddir \
-        PKG_CONFIG_PATH=/path/to/builddir/lib/pkgconfig
-    make && make check && make -C doc/latex
+- rust make -C mingw
+  - Downloads versions of libogg, opus, openssl.
+  - Compiles them.
+  - Compiles opusfile and examples against the built deps.
 - mkdir opusfile-${version}-win32
 - Copy AUTHORS COPYING README.txt include/opusfile.h to the release dir.
 - Merge changes between README.txt and the version in the last
@@ -49,8 +38,9 @@ Win32 binaries:
 - Copy .libs/libopusfile.dll.a to the release dir. (May not be needed?)
 - Copy ./lib/libopusurl-0.dll, etc. to the release dir.
 - Copy ./bin/*.ddl to the release dir for dependencies.
-- Copy any other dependent dlls, e.g. on Fedora 20 I needed to copy
+- Copy any other dependent dlls, e.g. on Fedora 23 I needed to copy
     /usr/i686-w64-mingw32/sys-root/mingw/bin/libgcc_s_sjlj-1.dll
+    /usr/i686-w64-mingw32/sys-root/mingw/bin/libwinpthread-1.dll
 - Copy doc/latex/refman.pdf to opusfile-${version}-win32/opusfile-${version}.pdf
 - Copy examples/.libs/*.exe to the release dir.
 - Strip *.dll *.a *.exe in the release dir.